The present invention relates to a comparator circuit, and more particularly to a comparator circuit having a hysteresis characteristic, wherein an input signal voltage causing an output voltage to be inverted from a first level to a second level is different from an input signal causing that to be inverted from the second level to the first level.
A conventional comparator circuit having a hysteresis characteristic comprises an amplifier having a first and second input ends and an output end, and a feedback circuit connected between the output and the second input ends of the amplifier to supply a comparison voltage corresponding to the voltage level at the output end. The first input end of the amplifier is supplied with an input signal. The feedback circuit comprises first and second resistors and a reference voltage source. The first and second resistors are series-connected between the output end of the amplifier and the reference voltage source. The connecting point between the first and second resistors is connected to the second input end of the amplifier.
The input signal voltage supplied to the first input end is compared with the comparison voltage at the second input end by the amplifier to produce an output voltage of first or second level at the output end of the amplifier in response to the relationship of the input signal voltage to the voltage at the second input end. Since the second input end of the amplifier is connected to the connecting point between the first and second resistors which are series-connected between the output end and the reference voltage source, the second input end is supplied with a first or second threshold voltage determined by the first and second levels of the output voltage at the output end. In other words, the comparator circuit has first and second threshold voltages different in voltage level from each other. In consequence, there is a difference in voltage between a first input signal voltage causing a change of the output voltage from the first level to the second level and a second input signal voltage causing a change of the output voltage from the second level to the first level. The voltage difference between the first and second input signal voltages (i.e., the voltage difference between the first and second threshold voltages) is called as a "hysteresis voltage". The hysteresis voltage determines a voltage range of the input signal voltage which does not change the output voltage after the output voltage is changed.
However, the first and second levels of the output voltage depend on the power supply voltage supplied for actuating the comparator circuit. In other words, the first and second threshold voltages of the comparator circuit have a dependence on the power supply voltage. As a result, the hysteresis voltage is also changed in response to the variation in the power supply voltage. The value of the hysteresis voltage is determined by the resistance ratio between the first and second resistors as well as the power supply voltage. If the resistance ratio is increased, the hysteresis voltage is decreased. If a high power supply voltage is supplied to the comparator circuit, the small hysteresis voltage can be obtained by increasing largely the resistance ratio. However, the resistance ratio between two resistors actually formed in a semiconductor integrated circuit device has a deviation from a value designed therefor, and the deviation becomes larger as the resistance ratio is larger. Moreover, a resistor having a high resistance value occupies a large area on the semiconductor substrate and is not suitable for an element on the semiconductor integrated circuit device. Consequently, the deviation of the hysteresis voltage becomes large, and the advantages of the formation of the comparator circuit as a semiconductor integrated circuit are lost.